Fig. 1.

Description of experiment. A: description of the task: following 4 brief preparatory taps over a specified target, the subject executed a last tap where after making contact with the target, the subject ramped up the vertical force applied by his finger to a maximum. The figure shows this last tap. The time of interest was 500 ms before and after contact (red in the time axis). The subject is wearing a custom thimble during the taps; the thimble allowed a unique contact point and friction cone for force direction. B: fingertip force in the 3 directions was recorded while performing the task; the figure shows the vertical force magnitude for 1 trial 500 ms before and after contact. C: fine-wire EMG from the 7 muscles of the index finger was recorded while performing the task. Normalized weighted electromyograph (EMG) is obtained after being filtered, full-wave rectified, normalized (by maximal voluntary contraction), weighted (by physiological cross-sectional areas), and smoothed (50-ms symmetric moving average) the EMG signals. D: the norm weighted EMG for each of the muscles, constitute a component of the muscle coordination pattern 7-dimensional (7D) vector [m(t)]. The coordination pattern alignment is the angle between the m(t) for each interval of time (−500 to 500 ms) and the m(t) at the time when the vertical force reached its maximum value. E: the best fit that described the evolution of the coordination pattern alignment during the transition from motion to force is a sigmoid.